Are hazards due to earthquakes in the New Madrid Seismic Zone overestimated

Are hazards due to earthquakes in the New Madrid Seismic Zone
overestimated?

Viewpoint:
Yes, major earthquakes in the Midwest are rare, and the geological
features of the NMSZ are quite different from the earthquake-prone regions
of the West Coast.

Viewpoint:
No, scientific understanding of earthquake phenomena is still limited,
and the large earthquakes of 1811 and 1812 suggest that the threat is
genuine.

People have a special dread of earthquakes. They strike without warning,
and can in the worst circumstances in a few moments kill thousands of
people and leave thousands of others injured and homeless. Hurricanes can
be equally devastating, but their arrival is usually predicted hours or
even days in advance, giving people the chance to flee or prepare
themselves. Tornadoes give less warning than other storms, but their
damage is typically very localized. Floods, avalanches, forest fires, and
other natural catastrophes wreak havoc with lives and property, but do not
present the special risks—or generate the fear—associated
with earthquakes. In the developed world, earthquakes are treated as a
unique risk to life and property, and in the regions most vulnerable to
damaging seismic events, construction of buildings, roads, and other
structures are strictly regulated to minimize damage and loss of life in
the event of a severe earthquake.

Quakeproof construction comes at significant cost. Cities where the
earthquake risk is highest—Tokyo, San Francisco, Los
Angeles—have had the greatest success in regulating new
construction to high safety standards and in retrofitting older structures
to improve safety. In the immediate aftermath of an event such as the
earthquake that struck near San Francisco in 1989, resistance to the costs
of improving safety in the effected area is naturally diminished. But even
in the places where earthquakes are a constant, present danger, funds for
quakeproof construction are far from unlimited. Engineering decisions are
always made in the context of balancing costs and benefits, and protection
from earthquakes is no exception.

At the opposite extreme from the quake-sensitive cities of California are
large urban centers such as New York and Chicago. A significant earthquake
near either of these cities would create a catastrophe far greater than
any in recorded history. Both cities house millions of people in
unreinforced masonry buildings that are very likely to collapse during an
earthquake. Bridges, tunnels, and mass transportation systems are also not
constructed to meet earthquake standards. Large residential and commercial
neighborhoods rest upon reclaimed land, or "landfill,"
likely to liquefy during a quake. But while the risk presented by an
earthquake in these cities is enormous, the chance of an earthquake
striking either place is, while not zero, very low, and the governments
and citizens of New York and Chicago have chosen to treat the threat as
negligible.

The region in the Mississippi River Valley known as the New Madrid Seismic
Zone presents a more complicated equation of risk and benefit. In
the early nineteenth century, New Madrid, Missouri, was the epicenter of
some of the largest earthquakes ever recorded in the continental United
States. The region was sparsely populated at that time, but historical
accounts describe houses falling down in New Madrid, chimneys toppling
near St. Louis, and—perhaps apocryphally—church bells
ringing as far away as Boston. Because the region is much more heavily
populated today, a large earthquake there could cause great death and
destruction.

But engineers, scientists, and civic planners have not fully accepted the
conclusion that the New Madrid region faces the same earthquake threat as
the more seismically active cities on the West Coast. There were no major
seismic events in the region during the twentieth century, and those
lesser events that occurred (mostly earthquakes of magnitude 5 or less)
did virtually no damage. While the quake-prone regions of California rest
on or near active fault lines, the geology of the New Madrid region is
more complicated, and scientists disagree about its risk profile for the
future. More scientific study is underway, and will no doubt continue into
the future. In the meantime, decisions about how to incorporate seismic
risks into decisions about construction and civic planning will have to be
made with incomplete scientific evidence, and the worrisome historical
record of a massive quake long ago will have to be measured against the
complacency inspired by nearly two hundred quake-free years.

—LOREN BUTLER FEFFER

Viewpoint: Yes, major earthquakes in the Midwest are rare, and the
geological features of the NMSZ are quite different from the
earthquake-prone regions of the West Coast.

Scientists face a challenge when discussing natural hazards with the
public. On the one hand, experts want to warn to public about the hazard
and convince citizens to take appropriate steps to mitigate it. On the
other hand, overestimating the hazard can have either of two undesirable
effects. If the general public does not find the warning credible, the
warning will be ignored. Alternatively, accepting an overestimate of the
hazard may divert time and resources from other goals that could result
in more societal good.

The Cost of Stringent Building Codes

An example of this challenge is illustrated by attempts to estimate the
seismic hazard for parts of the central United States due to earthquakes
in the New Madrid Seismic Zone (NMSZ). Recent United States Geological
Survey maps predict that the seismic hazard in the area is surprisingly
high, in some ways exceeding that found in California. Because most
earthquake-related deaths result from the collapse of buildings—a
principle often stated as "earthquakes don't kill people;
buildings kill people"—the primary defense against
earthquakes is designing buildings to ensure the safety of the
inhabitants. As a result, authorities in Memphis, Tennessee, and other
portions of the area are currently considering adopting new building
codes that require that buildings be built to the stringent standards of
earthquake resistance used in California. Given that these requirements
are expensive and might raise building costs by 10 to 35%, it is natural
to ask whether the hazard estimate is realistic. Similarly, it is
important to consider whether these costs are justified by the resulting
increases in public safety or whether alternative, less expensive
strategies might better balance costs and benefits.

To consider these questions, it is important to recognize that a
community's choice of building codes in earthquake-prone areas
reflects a complicated interplay between seismology, earthquake
engineering, economics, and public policy. The issue is to assess the
seismic hazard and choose a level of earthquake-resistant construction
that makes economic sense given that such a design raises construction
costs and diverts resources from other purposes. For example, money
spent making schools earthquake-resistant is therefore not available for
hiring more teachers, improving their pay, or providing better class
materials. Similarly, funds spent making highway bridges
earthquake-resistant cannot be used for other safety improvements
addressing more common problems. Thus, ideally building codes should not
be too weak, permitting unsafe construction and undue risks, or too
strong, imposing unneeded costs and encouraging their evasion. Deciding
where to draw this line is a complex policy issue for which there is no
definite answer.

Hazards and Risks

In assessing the potential danger posed by earthquakes or other natural
disasters it is useful to distinguish between hazards and risks. The
hazard is the intrinsic natural occurrence of earthquakes and the
resulting ground motion and other effects. The risk is the danger the
hazard poses to life and property. The hazard is a geological issue,
whereas the risk is affected by human actions.

Unfortunately, earthquake risks are not well understood: earthquake risk
assessment has been described by one of its founders as "a game
of chance of which we still don't know all the rules."
Nowhere is this more the case than in the New Madrid Seismic Zone, where
the situation is quite different from the familiar case of California,
as shown in Figure 1. Earthquakes in California occur as part of the
plate boundary zone that spans most of the western United States and
that accommodates most of the approximately 45 mm/year net motion
between the Pacific and North American plates. (A plate is a layer of
rock mass both within the earth and on its crust.) In contrast, the NMSZ
is within the generally stable interior of the North American plate,
which measurements from the Global Positioning System (a collection of
military satellites in known orbits) show deforms by less than 2
mm/year. Hence, major earthquakes are far more common in California:
large earthquakes (magnitude 7 or greater) taking up the interplate
motion typically occur on major faults in California about every 100 to
200 years, whereas the small intraplate deformation in the NMSZ appears
to give rise to earthquakes of this size about every 500 to 1,500 years.

As a result, high-magnitude earthquakes in the Midwest are relatively
rare. Since 1816, there are thought to have been 16 earthquakes with
magnitude greater than 5 (about one every 10 years), and two with
magnitude greater than 6, or about one every 100 years. These have
caused some damage but have been considered more of a nuisance than a
catastrophe. For example, the largest NMSZ earthquake in the past
century, the November 9, 1968 (magnitude 5.5) southern Illinois
earthquake, was widely felt and resulted in some damage but caused no
fatalities. However, a repetition of the large earthquakes that occurred
from 1811 to 1812 in the NMSZ would be very damaging and likely cause
deaths. Historical accounts show that houses fell down in the tiny
Mississippi river town of New Madrid and that several chimneys toppled
near St. Louis. This data implies that these earthquakes had magnitude
about 7.2 and provide insight into the effects of future ones.

The risk that earthquakes pose to society depends on how often they
occur, how much shaking they cause, and how long a time interval we
consider. Figure 2 illustrates this idea for California and the New
Madrid Seismic Zone, using circles representing approximate areas within
which an earthquake might significantly damage some buildings.
Earthquakes of a given magnitude occur 100 times more frequently in the
NMSZ than in California. However, because seismic energy is transmitted
more efficiently by the rock in the Midwest, NMSZ earthquakes shake
about the same area as California earthquakes one magnitude unit
smaller. Thus, in 100 years much of California will be shaken seriously,
whereas only a much smaller fraction of the NMSZ would be. After 1,000
years, much of the NMSZ has been shaken once, whereas most of the
California area has been shaken many times.

Guarding Against the Greatest Possible Hazard—Too Costly?

Typically, buildings have useful lives of about 50 to 100 years. Thus,
buildings in the Midwest are unlikely to be shaken during their
lifetimes, whereas buildings in California are likely to be. Given this
difference, several approaches are possible. One, to

assume the highest possible hazard, is taken by the United States
Geological Survey seismic hazard maps, which show the New Madrid zone as
the most seismically hazardous in the nation. This estimate assumes that
the 1811-1812 earthquakes were, and hence the largest future earthquakes
will be, magnitude 8 earthquakes, much larger than other lines of
evidence suggest. The maps further assume that ground motions from these
earthquakes would significantly exceed that expected from previous
models based on recorded shaking. Hence, the maps predict that the
ground motion expected in 50 years at 2% probability, or roughly every
2,500 (50/0.02) years, for the New Madrid zone exceeds that for San
Francisco or Los Angeles. These estimates have considerable uncertainty
because the underlying physical cause of the earthquakes is unclear, the
magnitudes and recurrence times for the largest earthquakes are
difficult to infer, and the likely ground motion from such earthquakes
is essentially unconstrained. Nonetheless, these estimates have led to
the proposal that buildings in the Memphis area be built to California
codes. This option maximizes safety at considerable cost because
buildings in Memphis are much less likely to be seriously shaken. Given
that this option is based on a very high estimate of the hazard, it
seems desirable to explore alternatives. For example, the code might
require that buildings be built to an intermediate standard, perhaps the
ground motion expected once in 500 years. The values are less uncertain
than the 2,500 year ones because they are closer to what has actually
been seismologically recorded. This approach seems likely to be more
realistic, given how rare major earthquakes are, and gives reasonable
seismic safey at significantly lower cost. As a result, it seems
premature to adopt California-style codes before a careful analysis of
costs and benefits.

Inform the Public with Clear and Accurate Data

This discussion indicates that deciding how to address earthquake
hazards in the Midwest is best done by taking a balanced approach of
recognizing the hazard while not exaggerating it. Although the public
and news media are drawn by images of destruction during earthquakes, it
is useful to bear in mind that in the United States earthquakes have
claimed an average of nine lives per year nationally, putting
earthquakes at the level of risk equal with in-line
skating or football, but far less than bicycles. To date, the deaths
are primarily in the western United States: earthquakes in the central
United States have caused no fatalities for more than 150 years.
Unfortunately, official statements and publications intent upon trying
to interest the public in NMSZ seismic hazards often use inflated
language, for example referring to future magnitude 6 earthquakes, which
are unlikely to cause fatalities or major damage, as
"devastating". Most such pronouncements do not convey how
rare major Midwest earthquakes are (a major earthquake anywhere in the
six-state seismic zone is expected less than once every 100 years on
average) and do not explain how different the situation is from
California. As a result, the impressions given are misleading and may
lead to poor policy choices. This is especially unfortunate because the
public, media, and authorities have less sophistication in understanding
earthquake issues than their counterparts in California. A more sensible
way to present seismic hazards would be to be candid with the public so
that evaluation of the costs and benefits of alternative policies can be
done with as much information on the issues and uncertainties as
possible. It would be wise to follow American physicist Richard
Feynman's 1988 admonition after the loss of the space shuttle
Challenger
: "NASA owes it to the citizens from whom it asks support to be
frank, honest, and informative, so these citizens can make the wisest
decisions for te use of their limited resources. For a successful
technology, reality must take precedence over public relations, because
nature cannot be fooled."

—SETH STEIN

Viewpoint: No, scientific understanding of earthquake phenomena is still
limited, and the large earthquakes of 1811 and 1812 suggest that the
threat is genuine.

There is only one thing wrong with the notion that the earthquake hazard
in the New Madrid Seismic Zone (NMSZ) is
overestimated—geophysicists, geologists, seismologists, and other
experts in this field do not know enough about how to predict
earthquakes even to make this statement. This is especially true of
earthquakes that happen in the Mississippi River Valley. No one knows
this better than Arch Johnston, director of the Center for Earthquake
Research and Information at the University of Memphis, and coordinator
for the Hazards Evaluation Program at the Mid-America Earthquake Center.
According to Johnston, New Madrid released more seismic energy in the
nineteenth century than did the entire western United States. "In
the twentieth century," however, as Johnston reported in
Seismicity and Historical Background of the New Madrid Seismic Zone,
"the fault zone has been relatively quiescent, with only a few
minor-damage events exceeding magnitude 5… . Understanding which
century is more representative of normal New Madrid behavior is perhaps
our greatest current challenge."

Plate Tectonics

One reason it's a challenge is that the New Madrid Seismic Zone
is not like other earthquake-prone regions. Most earthquakes on the
planet happen at the boundaries between tectonic plates, or land masses.
This plate tectonics model of the earth's surface emerged in the
1960s and gave geoscientists a way to understand previously baffling
things like the origin of mountain ranges and oceans, volcanic
eruptions, and earthquakes. According to the model, the massive, rigid
plates are 30 to 90 miles thick and 12 to 124 miles wide. They move at
different speeds and directions on a hot, soft layer of rock called the
asthenosphere, whose currents move the plates a few inches a year.
Continents and oceans hide plate borders, but it is not hard to pinpoint
their locations. Where plates ram together, mountains and volcanoes
form; where they crack apart into yawning rifts, oceans are born; where
they grind past each other, the land is racked by earthquakes. Though
the plate tectonics model has been widely accepted by geophysicists, it
does not explain everything about earthquake behavior and
predictability.

Questions about Seismic Activity in the NMSZ

In the winter of 1811-1812, three massive earthquakes struck the
Mississippi River Valley, which is more than 1,000 miles from the
nearest plate boundary. These intraplate (or, within plate) earthquakes
were magnitude 8.2, 8.1, and 8.3, making them the most energetic
earthquakes ever recorded in the contiguous 48 states. The quakes were
not an isolated event. Researchers now have evidence that strong
earthquakes occurred in the NMSZ repeatedly in the geologic past. And
smaller earthquakes still shake the region—two to three
earthquakes a week, with one or two a year large enough for residents to
notice—making the central Mississippi Valley the most seismically
active region east of the Rocky Mountains. "According to current
scientific understanding, these were earthquakes where they
shouldn't be," Johnston wrote. "They are
outstanding examples of rare major-to-great earthquakes that happen
remote from the usual tectonic plate boundary or active intraplate
seismic zones… . Our understanding of the faulting process and
repeat times of New

One big—and fundamental—question is why an area in an
otherwise stable continent would release such massive amounts of seismic
energy? Another is, why are faults in an ancient rift system in the
crust under the Mississippi River valley, inactive for millions of
years, now becoming active? The proposed answers to these questions are
varied and speculative.

Perhaps the biggest question is when will the next large New Madrid
quake strike? A study in 1992 suggested that it might occur within the
next few hundred years. By December of 1998, however, at a meeting of
the American Geophysical Union, researchers who had monitored the New
Madrid region for signs of strain said the next significant earthquake
would not occur for another 5,000 or 10,000 years, if then.

Complicating matters is the fact that geo-physicists do not understand
why the New Madrid faults ruptured in the first place, leading
geophysicist Paul Segall of Stanford University to tell
Science
magazine that another magnitude 7 to 8 "can't be
dismissed at this point." Segall said strain may have
reaccumulated rapidly in the first century after 1812 and is now slowly
building toward another quake in the next century or two. Future Global
Positioning System (GPS) surveys should show whether the land is
motionless, or is slowly inching toward the next big earthquake.
"The simplest assumption is, if [big quakes] happened in the
past, they can happen in the future," Segall said.

In 1992, Segall, geophysicist Mark Zoback of Stanford, and colleagues
compared a 1991 GPS survey with a nonsatellite 1950s land survey and
calculated that ground near the fault was moving 5 mm to 8 mm a year.
Later, Zoback reported in
Science
magazine that he and his colleagues "made a detailed study in
1991 of crustal strain with the use of a dense concentration of geodetic
stations located astride a single major fault. Our repeated GPS
measurements of this network in 1993 and 1997 appear to indicate lower
rates of strain accumulation than we originally reported." But
lower strain rates do not necessarily imply lower seismic hazard for the
region, Zoback said, adding that it is possible that the strain energy
released in the storm of large earthquakes in the New Madrid area over
the past few thousand
years took hundreds of thousands, or even millions of years to
accumulate. "If this is the case, a slow rate of strain
accumulation over the past 6 years does not imply low seismic
hazard," Zoback said. "The persistently high rate of
seismic activity in the New Madrid seismic zone over the past few
thousand years implies high seismic hazard in the foreseeable future. To
communicate any other message to the public would seem to be a
mistake."

Can Earthquakes Really Be Predicted?

Such uncertainty about earthquake behavior and prediction extends well
beyond the New Madrid Seismic Zone. In February 1999, the journal
Nature
sponsored a seven-week online debate among top researchers in the field
about whether the reliable prediction of individual earthquakes was a
realistic scientific goal. According to moderator Ian Main—a
reader in seismology and rock physics at the University of Edinburgh and
associate editor for the
Journal of Geophysical Research
—all the debate contributors who expressed an opinion
"agree that the deterministic prediction of an individual
earthquake, within sufficiently narrow limits to allow a planned
evacuation program, is an unrealistic goal." Another contributor,
Max Wyss of the Geophysical Institute at the University of
Alaska-Fairbanks, said, "The contributions to the debate about
earthquake prediction research in
Nature
so far clearly show that we have hardly scratched the surface of the
problem of how earthquake ruptures initiate and how to predict
them… . If the current knowledge of the earthquake initiation
process is so poorly founded that experienced researchers can maintain
the profound differences of opinion present in this debate," he
added, "we are in desperate need of the major research effort
that is not at present being made."

Hazards Emanating from Outside the NMSZ

There is one more problem with minimizing the earthquake hazard in the
New Madrid Seismic Zone—this zone may be the most recognized
source of earthquakes in the central Mississippi Valley, but it is not
the
only
source of seismic hazards. Some scientists estimate a 9-in-10 chance
that a magnitude 6 or 7 earthquake will occur in the New Madrid Seismic
Zone in the next 50 years. Others say a large earthquake will not strike
the region for at least 1,000 years, or even 5,000 or 10,000 years. Even
the new United States Geological Survey National Seismic Hazard Maps are
based on a 1,000-year cycle for magnitude-8 earthquakes in the New
Madrid Seismic Zone. But earthquakes occur in a range of magnitudes, and
the New Madrid Seismic Zone is not the only source of earthquakes in the
Mississippi River valley. In fact, the most recent earthquakes have come
from disturbances outside that zone.

In 1968, a magnitude 5.5 earthquake struck whose epicenter was 120 miles
southeast of St. Louis. In 1978 another earthquake rattled St. Louis,
this one measuring 3.5 on the Richter scale. The source was the St.
Louis Fault, discovered in 1948, a 45-mile-long fault running from
Valmeyer, Missouri, to Alton, Illinois. In 1990 a magnitude 4.6
earthquake originated south of Cape Girardeau, Missouri, in the Benton
Hills, a five-mile-wide forested incline in an otherwise flat alluvial
plain. It rattled seven states, and the epicenter was near New Hamburg
in Scott County, Missouri, which is outside the New Madrid Seismic Zone.
After the earthquake, the Missouri Department of Natural Resources (DNR)
and the USGS decided to investigate the source of seismicity in the
Benton Hills.

One of the few places in the central Mississippi Valley where faults
break the earth's surface is near the town of Commerce, Missouri.
Here, starting in 1993, geologist David Hoffman, an earthquake
specialist in the Division of Geology and Land Survey for the Missouri
DNR, excavated a dozen or more 15-foot-deep trenches in the Benton Hills
uplands and uncovered folds in the soil layers. The folds told Hoffman
that seismic activity had occurred there within the last 10,000 years
and suggested that the faults were still active. The site lies along a
linear magnetic feature called the Commerce geophysical lineament. Some
geo-physicists believe this northeast-trending lineament may represent a
fault zone more than 150 miles long. It is still poorly understood, but
this geologic feature could pose another large seismic threat to the
region. "There are lots of faults in the area, and it might
[indicate] the potential for earthquakes in the future," Hoffman
told the
St. Louis Riverfront Times
in 1999. "It takes studies of lots of these kinds of places and
features and compiling all the data and analyzing it … to get the
whole picture. We .. . have one piece of the puzzle with this spot.
We're the primary ones that have even looked here. Then
there's the rest of the state that hasn't been looked
at."

Still Much to Learn

Assessing the seismic risk in the Mississippi Valley is tricky because
faults and other geologic structures were deeply buried over hundreds of
millions of years by thick layers of sediment, and no clues to the
area's seismicity exist at the earth's surface. Other
seismic authorities agree with Hoffman that there is still a great deal
to be learned about the structure of the earth's crust under the
Mississippi River valley—and everywhere else. Even in northern
California, where the faults are much closer to the surface,
seismologists did not know about the fault that caused the magnitude
6.8, 1994 Northridge earthquake—57 dead; 9,300 injured; 13
billion dollars in damages;
50,000 homeless; 13,000 buildings destroyed—until the ground
started moving.

Using geophysical techniques and a growing array of tools, researchers
have gained substantial knowledge over the years about the nature of
earthquakes and the processes that drive them. Beginning in 1995, the
National Science Foundation's (NSF) Division of Earth Sciences
announced that it would fund research opportunities in a special area of
emphasis—tectonically active systems of the earth's
continental crust. NSF describes the initiative, called Active
Tectonics, as "a concerted attack on issues pertaining to the
connectedness of individual tectonic elements and physical-chemical
processes within a system, and how these relationships evolve through
time." The research is supposed to cross traditional geological
and geophysical disciplinary boundaries and link fields such as
earthquake seismology, basin analysis, geodetics, structural geology,
paleoseismology, geomorphology, reflection seismology, petrology,
regional tectonics, solid-earth geophysics, geochronology, geomagnetics,
rock mechanics, hydrology, tectonophysics, Quaternary studies,
paleomagnetics, volcanology, and more. The result, in the future, will
be better ways to define problems and approaches, better
characterization of the dynamic state of lithosphere and asthenosphere,
better 3-D resolution of regional tectonic architecture, integration of
historical geological facts and interpretations of a region's
tectonic evolution, and better ways to mitigate earthquake hazards.

In the meantime, earthquakes generated by hidden and poorly understood
geologic features will keep shaking the central Mississippi Valley.
Those who presume to know such things have different opinions about when
they strike—in 50 or 1,000 years? Never, or tomorrow? At the
Seismological Laboratory at the California Institute of
Technology-Pasadena, geophysicist Hiroo Kanamori likes to say that there
is only one known method to date for testing earthquake
predictions—wait and see.

Stover, Carl, and Jerry Coffman.
Largest Earthquakes in the United States.
Abridged from
Seismicity of the United States, 1568-1989.
USGS Professional Paper No. 1527. Washington, D. C.: United States
Government Printing Office, 1993.

KEY TERMS

ALLUVIAL PLAIN:

An assemblage of sediments marking the place where a stream moves from a
steep gradient to a flatter gradient and suddenly loses transporting
power. Typical of arid and semi-arid climates but not confined to them.

ASTHENOSPHERE:

The weak or soft zone in the upper mantle of the earth just below the
lithosphere, involved in plate movement and isostatic adjustments. It
lies 70 to 100 km below the surface and may extend to a depth of 400 km.

EPICENTER:

The point on the earth's surface that is directly above the focus
(center) of an earthquake.

GLOBAL POSITIONING SYSTEM (GPS):

An array of military satellites in precisely known orbits. By comparing
minute differences in arrival times of a satellite's radio signal
at two sites, the distance between markers tens of kilometers apart can
be determined to within a few millimeters.

LITHOSPHERE:

The rigid outer shell of the earth. It includes the crust and upper
mantle and is about 100 km thick.

MAGNITUDE:

A measure of the strength of an earthquake based on the amount of
movement recorded by a seismograph.

PLATE TECTONICS:

A theory of global tectonics according to which the lithosphere is
divided into mobile plates. The entire lithosphere is in motion, not
just the segments composed of continental material.

RICHTER SCALE:

A commonly used measure of earthquake magnitude, based on a logarithmic
scale. Each integral step on the scale represents a 10-fold increase in
the extent of ground shaking, as recorded on a seismograph. Named after
American seismologist Charles Richter.

RIFT:

A valley caused by extension of the earth's crust. Its floor
forms as a portion of the crust moves downward along normal faults.

SEDIMENT:

Any solid material that has settled out of a state of suspension in
liquid.

User Contributions:

if there might be another seismic activity how much of west tennessee would be disrupted during an earthquake. i figure between the memphis and nashville has a high concentration of sand and sandstone mixed with water will tear up with enough shaking. the last big rain wiped out a lot of roads back awhile without any other forces tema did a poor job of notifying of the dangerous road condition.

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